1. Field of the Invention
The present invention relates to a technical field of a crystal oscillator of a constant-temperature type (hereinafter called a constant-temperature type oscillator), and in particular, to a constant-temperature type oscillator that detects an operational temperature of a crystal unit in real time.
2. Description of the Related Art
Constant-temperature type oscillators keep the operational temperatures of their crystal units constant. For example, the constant-temperature type oscillators can obtain a highly-stable oscillating frequency of 0.1 ppm or less or on the order of 1 ppb without causing frequency changes dependent on the frequency temperature characteristics. Therefore, the constant-temperature type oscillators are applied to base stations of communication facilities.
FIGS. 2A and 2B are diagrams for explanation of one example of a related-art constant-temperature type oscillator, in which FIG. 2A is a cross-sectional view thereof, and FIG. 2B is a schematic circuit diagram thereof.
The constant-temperature type oscillator shown in FIGS. 2A and 2B includes an oscillator circuit 1 and a temperature control circuit 2, and respective elements forming the oscillator circuit 1 and the temperature control circuit 2 are installed on a circuit substrate 4. The circuit substrate 4 is held by lead wires 6 projecting on the surface of a metal base 5, for example, and a metal cover 7 is bonded thereon by resistance welding. The oscillator circuit 1 is composed of a surface-mount crystal unit 3A, and a capacitor and an oscillating amplifier of an oscillating part. The oscillator circuit 1 is formed as a Colpitts type circuit with the surface-mount crystal unit 3A serving as an inductance component.
FIGS. 3A to 3C are diagrams for explanation of a related art surface-mount crystal unit, in which FIG. 3A is a cross-sectional view thereof, FIG. 3B is a bottom view thereof, and FIG. 3C is a plan view of a crystal element thereof.
As shown in the FIG. 3A, the surface-mount crystal unit 3A is configured so that a crystal element 9 formed as, for example, an AT-cut crystal element or an SC-cut crystal element is housed in a case main body 8 made of laminar ceramics, which is formed to be concave, and a metal cover 10 is bonded to hermetically encapsulate the crystal element 9. As shown in FIG. 3B, the case main body 8 includes mounting terminals 11 on an outer bottom face thereof that includes crystal terminals 11a on a set of diagonal corners, and ground terminals 11b on the other set of diagonal corners.
As shown in FIG. 3C, leading electrodes 17a, 17b and excitation electrodes 16a, 16b are formed on the crystal element 9. Both sides of one end of the leading electrodes 17a, 17b extending from excitation electrodes 16a, 16b of the crystal element 9 are firmly fixed to a crystal holding terminal 13 with an electrically conductive adhesive 12. Then, the crystal element 9 is electrically connected to the crystal terminals 11a on the outer bottom face of the case main body 8 through wiring paths including end face electrodes on the outer side face (not shown). The metal cover 10 is bonded to a metal ring 18 on an opening end face of the case main body 8 by seam welding, for example, and the metal cover 10 is electrically connected to the ground terminals 11b on the outer bottom face of the case main body 8 through wiring paths including through electrodes on the frame wall (not shown).
In the temperature control circuit 2, a detecting voltage by a thermistor 3Rth which resistance value changes according to an operational temperature of the surface-mount crystal unit 3A and a fixed resistor 3Ra and a reference voltage by fixed resistors 3Rb and 3Rc are applied to an input of an operational amplifier 30A to obtain a comparison voltage. The comparison voltage controls an output of a power transistor 3Tr, which supplies electric power to a heating resistor 3Rh that is composed of a chip resistor. Thereby, an operational temperature of the surface-mount crystal unit 3A is kept constant.
In a constant-temperature type oscillator having such a structure, the surface-mount crystal unit 3A, the heating resistor 3Rh, the thermistor 3Rth and the power transistor 3Tr are installed on a under surface of the circuit substrate 4 facing the metal base 5. These respective elements 3 are covered with heat conducting resin 14. Thereby, the surface-mount crystal unit 3A, the heating resistor 3Rh, the thermistor 3Rth and the power transistor 3Tr are thermally coupled.
FIGS. 4A and 4B are diagrams for explanation of another example of a related art constant-temperature type oscillator, in which FIG. 4A is a diagram of a partial wiring pattern of a circuit substrate thereof, and FIG. 4B is a diagram of a thermistor thereof. As shown in FIGS. 4A and 4B, a thermistor circuit terminal 15A, to which the first terminal electrode 15a of first and second terminal electrodes 15a, 15b provided on both end sides of the thermistor 3Rth is connected, is connected to crystal unit circuit terminals 11B, to which the ground terminals 11b of the surface-mount crystal unit 3A are connected. Thereby, an operational temperature of the surface-mount crystal unit 3A is directly detected. The respective elements 3 of the oscillator circuit 1 and the temperature control circuit 2 other than the surface-mount crystal unit 3A, the heating resistor 3Rh, the thermistor 3Rth and the power transistor 3Tr are installed on the top surface of the circuit substrate 4.
Incidentally, JP-A-2006-311496 and JP-A-2006-278793 each discloses a related art.
However, in a constant-temperature type oscillator having the above-described configuration, the ground terminals 11b of the surface-mount crystal unit 3A, to which the one end of the thermistor 3Rth is connected, involves no electrical connections to the crystal element 9. Thus, in a strict sense, the thermistor 3Rth is not in direct thermal contact with the crystal element 9. Accordingly, there has been a problem that, even if the one end of the thermistor 3Rth is electrically connected to the ground terminals 11b, it is not necessarily possible to directly detect an operational temperature of the crystal unit, i.e., an operational temperature of the crystal element 9.
Further, because the one end of the thermistor 3Rth is connected to the ground terminal 11b of the surface-mount crystal unit 3A, the ground terminal 11b is provided with an electric potential on the one end side of the thermistor 3Rth. Accordingly, there has been a problem that, at the time of loading the constant-temperature type oscillator on a set substrate, the ground terminals 11b cannot be connected to the ground pattern of the set substrate, which makes it impossible to ground the metal cover 10 of the surface-mount crystal unit 3A.